WordPress is the king of the web, for many people putting a webpage online is equal to deploying and adapting WordPress to the needs they have, but what does WordPress such an useful software for building almost any kind of webpage?
The work others did on building plugins and themes for WordPress and the GPLv2 or later license which helps them sharing that work.
You can find themes and plugins for almost any purpose making WordPress a Swiss Army Knife and the GPLv2 or later WordPress is licensed in states than those plugins and themes must be distributed under the license of WordPress itself without further restrictions.
This allow you to virtually adapt any theme or plugin which does not fit you completly to your needs in a complete way and then distribute your changes under the same license.
Does it apply to paid plugins/themes?
It may be possible a plugin or theme is not distributed under a GPLv2 or later compatible license but that would expose the author to be demanded by anyone who holds copyright on WordPress for violating their license.
But you can sell software copies GPLv2 or later programs so the most probable outcome is you being able to edit and further distribute any plugin or theme you buy created for their usage on WordPress.
I hope WordPress becomes everyday better and we can enjoy more years of this amazing software.